1. Field of the Invention
The present invention relates to a hydroponic nutrient solution control system for growing plants hydroponically by controlling supplies of nutrient solutions (solutions of inorganic fertilizers in water) to the plants.
2. Description of the Background Art
In the hydroponics, the plants are grown under the control of a hydroponic nutrient solution control system which controls supplies of nutrient solutions to the plants.
In a conventional hydroponic nutrient solution control system, this controlling is performed according to the pH and/or the electrolytic conductivity of the nutrient solutions.
For instance, when the electrolytic conductivity of the nutrient solution decreased, high concentration nutrient solutions are supplied to the nutrient solution in order to raise the electrolytic conductivity to prescribed target range, whereas when the electrolytic conductivity of the nutrient solution increased, water is added to the nutrient solution in order to lower the electrolytic conductivity to prescribed target range.
Similarly, in a case of using the pH, an acid solution and an alkali solution are used to control the pH of the nutrient solution within a prescribed target range.
However, in such a conventional hydroponic nutrient solution control system, it has been difficult to accurately supply the nutrient solutions containing the ingredients necessary for the plants to be grown. This is due to the fact that in a course of the growth of the plants, the organic acids are leaked from the roots of the plants, such that even when the ingredients unnecessary for the plants are being compiled, this fact is not reflected by the pH or the electrolytic conductivity of the nutrient solutions. As a consequence, the necessary adjustments of the nutrient solutions cannot be provided, so that the growth of the plants are retarded if not hampered fatally.
Also, because the information on the pH and the electrolytic conductivity cannot reflect the actual state of the nutrient solutions accurately, even when the abnormality was found in the growing plants, the cause of the abnormality cannot be ascertained from the information on the pH and the electrolytic conductivity, so that the entire nutrient solutions have to be replaced in such a case, which increases the waste of the nutrient solutions.
Also, in a conventional hydroponic nutrient solution control system, the controlling of the pH is performed by using one acid solution and one alkali solution only. However, necessary amounts of the acid solution or the alkali solution vary from time to time, so that it is desirable to have more than one acid solutions and more than one alkali solutions, in order to be able to cope with various different situations.
It is known that a plant in a process of growing may consume particular ingredients more than other ingredients, depending on the external conditions such as an amount of sunshine, a temperature of the air, and a temperature of the nutrient solution. In such a case, the nutrient solution control according to the pH or the electrolytic conductivity tends to cause a lack of particular nutrient solution ion ingredients. In this state, the plant is unable to absorb the necessary ion ingredients because of their low concentration in the nutrient solution.
On the other hand, the ion ingredients in the nutrient solution cannot be controlled easily, because the ions exist in forms of bases such as KNO.sub.3.
Furthermore, when the concentration of a particular kind of ions is very low, the balance of ions achieved in the original nutrient solution is often lost, such that the pH of the nutrient solution often differs greatly from the original value.